Methods of positioning and/or orienting nanostructures

ABSTRACT

Methods of positioning and orienting nanostructures, and particularly nanowires, on surfaces for subsequent use or integration. The methods utilize mask based processes alone or in combination with flow based alignment of the nanostructures to provide oriented and positioned nanostructures on surfaces. Also provided are populations of positioned and/or oriented nanostructures, devices that include populations of positioned and/or oriented nanostructures, systems for positioning and/or orienting nanostructures, and related devices, systems and methods.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to and claims priority fromProvisional U.S. Patent application Serial No. 60/370,113, filed Apr. 2,2002, which is hereby incorporated herein in its entirety for allpurposes.

BACKGROUND OF THE INVENTION

[0002] Nanotubes, nanocrystals, nanowires, and particularlysemiconductor nanowires have gained a great deal of attention for theirinteresting and novel properties in electrical, chemical, optical andother applications. Such nanomaterials have a wide variety of expectedand actual uses, including use as semiconductors for nanoscaleelectronics, optoelectronic applications in emissive devices, e.g.,nanolasers, LEDs, etc., photovoltaics, and sensor applications, e.g., asnano-ChemFETS.

[0003] While commercial applications of the molecular, physical,chemical and optical properties of these materials have been postulatedfor all of these different types of materials, generating commerciallyviable products has not, as yet, been forthcoming. In the world ofdevices with integrated nanomaterial elements, some of the difficultiesin producing commercially viable products has stemmed from thedifficulty in handling and interfacing with such small scale materials.Specifically, for the most part, these materials are produced in bulk asfree standing elements that must be positioned within an operationaldevice. Accurate and reproducible positioning of these materials hasproven difficult.

[0004] Accordingly, it would be desirable to be able to provide methodsof positioning and orienting nanowires on substrates or withinintegrated devices or systems, in a reasonably practicable fashion. Thepresent invention meets these and a variety of other needs.

SUMMARY OF THE INVENTION

[0005] The present invention is generally directed to methods ofpositioning and orienting nanostructures, and particularly nanowires onsubstrates for subsequent use, integration or application. The inventionalso envisions systems for practicing such methods, devices that includeoriented and positioned nanostructures, populations of positioned and/ororiented nanostructures, and systems that include such positioned and/ororiented nanostructures.

[0006] In one aspect, the present invention provides a method ofdepositing nanowires on a surface substantially in a desiredorientation. The method generally comprises flowing a fluid containingnanowires over the surface in a first direction, where the firstdirection is substantially parallel to a desired longitudinalorientation of the nanowires. The nanowires in the solution are thenpermitted to become immobilized onto the surface, with the longitudinaldimension of the nanowires being substantially oriented in the firstdirection.

[0007] In a further aspect, the invention is directed to methods ofpositioning nanowires in one or more predetermined regions on asubstrate. The methods typically comprise providing a substrate having afirst surface, overlaying the first surface with a mask, where the maskprovides fluid access to one or more first predetermined regions on thefirst surface, but not to one or more second predetermined regions onthe surface of the substrate. A fluid containing nanowires is thenflowed through the mask and into contact with the first predeterminedregions of the substrate surface. The nanowires contained in thenanowire containing fluid are then permitted to immobilize in the firstpredetermined regions of the surface of the substrate.

[0008] In another aspect, the invention is directed to one or morepopulations of nanowires immobilized on a planar surface of a substrate,where the population(s) of nanowires are substantially longitudinallyoriented in a first direction parallel to the planar surface.

[0009] Similarly, the invention includes populations of nanowiresimmobilized on a surface of a substrate that comprise a first set ofnanowires immobilized in a first selected region of the surface of thesubstrate, and a second set of nanowires immobilized in a secondselected region of the surface of the substrate, the second selectedregion being separate from the first selected region.

[0010] The invention is also directed to a nanowire based device thatcomprises at least a first population of nanowires immobilized in atleast a first region of a surface of a substrate, the first populationof nanowires being substantially longitudinally oriented in a firstdirection. The devices of the invention typically include at least firstand second electrical contacts disposed on the first region of thesurface of the substrate. The first and second electrical contacts aretypically separated from each other on the first surface of thesubstrate in the first direction by a less than an average length of thenanowires in the population of nanowires.

[0011] The invention also includes a substrate comprising a plurality ofpopulations of nanowires deposited upon a first surface of saidsubstrate, wherein each of the populations of nanowires is deposited andimmobilized in a separate discrete region of the surface of thesubstrate. In preferred aspects, electrical contacts are disposed in theseparate regions such that at least one wire in the populations ofnanowires bridges and connects at least two electrical contacts.

[0012] In a further aspect, the invention is directed to a system fororienting nanowires on a surface of a substrate in accordance with themethods of the invention. The system typically comprises a substratehaving a first surface, a fluid channel disposed on the first surface,and a fluid direction system coupled to the first channel and coupled toa source of fluid containing nanowires, for flowing the fluid containingnanowires in a first direction through the first fluid channel.

[0013] The invention also includes, as one aspect, a system forpositioning nanowires on a surface of a substrate. As above, the systemincludes the substrate having the first surface. A masking element isprovided over the first surface which provides fluid access to separatediscrete regions of the first surface of the substrate. A source offluid that includes the nanowires or other nanostructures is providedfluidicly coupled to the fluid passages in the masking element. A fluiddirection system is operably coupled to the fluid source and passages inthe masking element to delver the fluid from the source to the passages,so that the nanowires in the fluid can contact and thus be immobilizedupon the discrete regions of the surface of the substrate.

BRIEF DESCRIPTION OF THE FIGURES

[0014]FIG. 1 schematically illustrates a wafer based process forpositioning and orienting nanowires on a substrate.

[0015]FIG. 2 schematically illustrates the integration of electricalelements with positioned and oriented nanowires on a wafer substrate.

[0016]FIG. 3 is a schematic illustration of patterned substratefunctionalization followed by positioning and orienting of nanowires.

[0017]FIG. 4 shows a schematic illustration of bidirectional orientationof nanowires in accordance with the processes described herein.

[0018] FIGS. 5A-5D show schematic illustrations of different fluidicchannel structures designed to achieve different deposition patterns ofnanowires on substrate surfaces.

[0019]FIG. 6 is a schematic illustration of an overall system forpositioning and aligning nanowires onto substrate surfaces.

[0020]FIG. 7 is an SEM image of oriented nanowires immobilized on asubstrate surface.

[0021]FIG. 8A is a postulated electrode deposition over the orientednanowire population shown in FIG. 7, and FIG. 8B shows a plot ofexpected frequency of 0, 1, 2 and 3 wire connections between electrodepairs.

[0022]FIG. 9 shows aligned nanowires connected to electrical contactpairs.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention is generally directed to methods ofpositioning and/or orienting nanowires on substrates, nanowires sopositioned and/or oriented, devices produced from such oriented and/orpositioned nanowires, and systems used in so orienting and/orpositioning such nanowires.

[0024] As used herein, the term “nanowire” generally refers to anyelongated conductive or semiconductive material that includes at leastone cross sectional dimension that is less than 500 nm, and preferably,less than 100 nm, and has an aspect ratio (length:width) of greater than10, preferably, greater than 50, and more preferably, greater than 100.Examples of such nanowires include semiconductor nanowires as describedin Published International Patent Application Nos. WO 02/17362, WO02/48701, and 01/03208, carbon nanotubes, and other elongated conductiveor semiconductive structures of like dimensions. Particularly preferrednanowires include semiconductive nanowires, that are comprised ofsemiconductor material selected from, e.g., Si, Ge, Sn, Se, Te, B,Diamond, P, B—C, B—P(BP6), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,Cul, AgF, AgCl, AgBr, Agl, BeSiN2, CaCN2, ZnGeP2, CdSnAs2, ZnSnSb2,CuGeP3, CuSi2P3, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)2, Si3N4, Ge3N4,Al2O3, (Al, Ga, In)2(S, Se, Te)3, Al2CO, and an appropriate combinationof two or more such semiconductors. In certain aspects, thesemiconductor may comprise a dopant from a group consisting of: a p-typedopant from Group III of the periodic table; an n-type dopant from GroupV of the periodic table; a p-type dopant selected from a groupconsisting of: B, Al and In; an n-type dopant selected from a groupconsisting of: P, As and Sb; a p-type dopant from Group II of theperiodic table; a p-type dopant selected from a group consisting of: Mg,Zn, Cd and Hg; a p-type dopant from Group IV of the periodic table; ap-type dopant selected from a group consisting of: C and Si.; or ann-type is selected from a group consisting of: Si, Ge, Sn, S, Se and Te.

[0025] The present invention provides for the selective deposition ofnanowires in preselected regions of substrates by providing a maskinglayer that masks off certain portions of the substrate surface, whileproviding fluid access to those portions of the substrate surface whereit is desired to deposit nanowires. Fluid containing the nanowires isthen directed through the mask such that the nanowires contact thedesired regions of the substrate, and the nanowires are immobilizedthereon. In the context of the invention, the substrate to whichnanowires are immobilized may comprise a uniform substrate, e.g., awafer of solid material, e.g., silicon, glass, quartz, plastic, etc. orit may comprise additional elements, e.g., structural, compositionaletc. For example, the substrate may include other circuit or structuralelements that are part of the ultimately desired device. Particularexamples of such elements include electrical circuit elements such aselectrical; contacts, other wires or conductive paths, includingnanowires or other nanoscale conducting elements, optical and/oroptoelectrical elements, e.g., lasers, LEDs, etc., structural elements,e.g., microcantilevers, pits, wells, posts, etc.

[0026] By further controlling the direction of flow of the nanowirecontaining fluid through the mask, one can substantially align or orientthe nanowires that immobilize to the surface of the substrate. Inparticular, nanowires that are being deposited on a surface tend to belongitudinally oriented in the direction of flow of the carrier fluid inwhich they are suspended. Accordingly, one can substantiallylongitudinally orient nanowires on a surface by flowing the carrierfluid in a direction across the surface that is parallel to the desiredlongitudinal orientation of the nanowires. By “substantiallylongitudinally oriented” is meant that the longitudinal axes of amajority of nanowires in a collection or population of nanowires areoriented within 30 degrees of a single direction. Preferably, at least80% of the nanowires in a population are so oriented, more preferably atleast 90% of the nanowires are so oriented. In certain preferredaspects, the majority of nanowires are oriented within 10 degrees of thedesired direction.

[0027] In the context of the present invention, it is generallypreferred to provide for selective positioning of nanowires on certainregions of substrates while simultaneously providing for desiredorientation of those nanowires. However, as will be readily appreciated,there may be a number of instances in which one aspect of the inventionis more desired than the other. For example, in certain cases, it may bedesired to align nanowires on a substrate surface with little or noregard for the positioning of the nanowires on that surface. Similarly,there may be instances where selected positioning of nanowires isdesired with little or no regard for the orientation of the nanowiresonce positioned. Although the disclosure may focus a particulardiscussion on one aspect or the other, such discussion is generally forease of understanding and convenience of description. It will beappreciated that in many cases, such disclosure applies equally to allaspects of the invention.

[0028] As noted previously, selective contact between nanowires andregions of a substrate is accomplished through a mask-based method,where a masking element is placed over the entire substrate surface. Themask provides for fluidic access to the desired regions of thesubstrate. A mask may be a simple stencil type mask where a solid layerthat includes apertures disposed through it is placed or fabricated overthe surface of the substrate. The apertures provide the fluid access tocertain regions of the substrate surface and by directing fluidcontaining nanowires (or other collections of nanowires, e.g., drypowders, etc.) to the surface of the substrate, one can ensure nanowirecontact, and ultimately localization and immobilization to thoseregions. By way of example, nanowires may be particularly targeted forpositioning or localization in desired areas of the substrate, e.g.,areas in which integration with additional elements is to occur, or tokeep nanowires out of areas in which their presence could provedetrimental to ancillary functions of a nanowire based device. By way ofexample, it may be particularly desirable to ensure hat nanowirescontact electrical contacts or other circuit elements, while avoidingcontact with other regions of the substrate surface. By doing so, onecan ensure that efforts at depositing nanowires are focused in thoseregions of the substrate where deposition is desired, and not in otherregions where it is less desirable.

[0029] In particularly preferred aspects, however, the masking elementwill be somewhat more complex than a simple stencil like mask. Inparticular, in order to provide for both positioning and orientation ofnanowires on a substrate surface, it is generally desirable to providefor directed flow of fluid across the surface of the substrate. As such,it will generally be desired to provide a masking element that providesfluidic channels across selected regions of a substrate's surface. Suchmasking elements are also often referred to as manifolds. In brief, onecan fabricate one or more grooves into a planar substrate to provide amanifold element. This planar element is then mated with the substratesurface upon which selective deposition of nanowires is desired. Themating of the manifold element with the substrate surface encloses thegroove on the manifold element and provides a channel which includes asone of its walls, a portion of the substrate surface. The groovetypically includes a fluid inlet port and a fluid outlet port to permitthe introduction and flow of fluid containing nanowires into and throughthe channel in the manifold element. In more preferred aspects, thesefluidic channels disposed over the substrate surface will be microscalein cross section, e.g., having a width dimension across the substratethat is less than 1 mm, preferably, less than 500 μm, and in many cases,less than 100 μm.

[0030] One of the advantages of the present invention is its ability tobe readily adapted to provide methods for larger scale production ofnanowire containing devices by providing full wafer scale nanowireorientation and/or positioning processes. In particular, a nanowirecontaining device, like most integrated circuit devices, is typicallyembodied in a small chip. Like the integrated circuit industry, it wouldbe desirable to be able to manufacture multiple devices in parallel fromindividual and larger wafers.

[0031] In accordance with the present invention, a substrate wafer isprovided from which multiple devices are to be produced. A fluid thatcontains nanowires is contacted with, or in the case of flow alignednanowires, flowed over all or one or more selected portions of thesubstrate in a desired direction. As noted above, for flow alignednanowires, the direction of flow will typically dictate the substantiallongitudinal orientation of the nanowires that become immobilized on thesurface of the substrate.

[0032] In some cases, it is preferred to provide nanowires only onselected regions of the surface of the substrate, e.g., to minimizeextraneous wire deposition, avoid wasting wires on unused portions ofthe substrate, etc. In such cases, a masking element is provided overthe substrate surface to ensure that nanowire containing fluid onlycomes into contact with one or more selected regions on the substratesurface. By way of example, a channel block or manifold that includesone or more channel grooves is placed against the substrate wafer andfluid containing the nanowires is flowed through the channels in thedesired direction to provided oriented nanowires in the selectedlocations on the wafer surface.

[0033] In order to produce multiple devices from the single wafer,multiple electrical interface components are provided on the wafer. Forexample, in producing multiple simple devices that includes twoelectrical contacts bridged by one or more nanowires, multiple pairs ofelectrical contacts can be provided on the substrate wafer,corresponding to each device. Typically, each pair of such electricalcontacts will be provided close enough to each other in the desireddirection, e.g., the direction of longitudinal orientation of thenanowires, such that a nanowire could bridge the space between theelectrical contacts. Multiple pairs of electrical contacts are providedat multiple different positions on the wafer surface. The wafercontaining multiple nanowire devices, e.g., one or more nanowiresbridging a pair of electrodes, are then segmented into multiple separatedevices.

[0034]FIG. 1 illustrates a wafer based process for producing nanowirebased devices in which nanowires are oriented, positioned and integratedwith electrical contacts for subsequent application. As shown, asubstrate wafer 100 is provided from which multiple nanowire baseddevices are to be fabricated. The wafer 100 may include surfacefunctionalization, e.g., as described herein. A masking element, such asmanifold 102 is overlaid on the relevant surface of the wafer 100. Themanifold includes a plurality of fluid accesses, e.g., fluid channels104-118, to the surface of the substrate. In particular, as shown,channels 104-118 are sealed on one side by the surface of the wafer 100,as described above. These channels are coupled to fluid ports 120 and122 as the fluid inlet and outlet ports, respectively.

[0035] The manifold or masking element 102 may take on a variety offorms and/or be fabricated from a variety of materials. By way ofexample, the manifold may be fabricated from rigid substrates, e.g.,glass, quartz, silicon, or other silica based materials. Such materialsprovide ease of manufacturing, in that the elements of the manifold,e.g., the fluidic channels, can be fabricated by processes that are wellknown in the microfluidics, and microfabrication industries, e.g.,photolithography and wet chemical etching. Similarly, polymericmaterials may be used and are readily manufactured using micromoldingtechniques, e.g., injection molding, microembossing, etc. In somepreferred cases, flexible materials are desirable as they provideenhanced contact between the manifold element and substrate surfacesthat may not be perfectly flat. Examples of such materials include,e.g., polydimethylsiloxane (PDMS) and the like. Such materials arereadily produced by micromolding techniques, where molds are fabricatedin accordance with well known microfabrication techniques, e.g.,photolithography and nickel electroforming of a master, followed by insitu polymerization of the PDMS manifold. Methods of fabricating suchmanifolds from a large variety of different materials are described indetail in the microfluidic patent literature, e.g., U.S. Pat. No.,6,180,239 to Whitesides et al, and U.S. Pat. No. 5,500,071 to Swedberget al., and U.S. Pat. No. 6,123,798 to Ghandi et al., the fulldisclosure of each of which are hereby incorporated herein by referencein their entirety for all purposes.

[0036] A fluid that contains nanowires is flowed into fluid inlet port120 and through channels 104-118, and out of fluid port 122. During flowof the fluid, nanowires immobilize to the regions of the wafer surfacethat are included in the channels 104-118. Removal of the manifold thenyields immobilized nanowire populations 124-138 in selected regions thatcorrespond to the regions accessed by the fluid. Further, because thefluid was flowed in a selected direction, e.g., through the channelsfrom the inlet port to the outlet port, the nanowires that areimmobilized on the wafer surface in these selected regions aresubstantially longitudinally oriented in the direction of fluid flow, asshown by expanded view 140 which shows oriented individual wires 142.

[0037] In alternate aspects, one may employ more complex fluidicchannels or fluid control systems in or attached to the manifold elementto more acutely control where and how nanowires immobilize on thesurface of the substrate during the deposition process. In particular,one can take advantage of fluid mechanics within the manifold channelsin order to more precisely direct deposition of nanowires against asubstrate surface.

[0038]FIG. 5 provides examples of channel geometries or control systemsthat can be used to provide for wire deposition in desired locations. Byway of example, FIG. 5A shows a cross section of a fluidic channel 500,viewed from above where the fluid channels widen at the region 502 ofdesired deposition. By widening the channels, fluid velocity throughthat channel portion is slowed (the residence time of wires in thisregion is increased) which enhances the likelihood that wires willdeposit against the substrate in these regions, e.g., shown as thedashed oval 504. Alternatively, as shown in FIG. 5B, channel regions 510that have regions with shallower depths 512, e.g., shorter diffusiondistances required to be traversed to reach the substrate surface, maybe provided. By providing a shorter diffusion distance between the fluidand the substrate surface region of interest, e.g., region 514, again,one may enhance the rate at which wires contact and are deposited on thedesired surface regions.

[0039] In other aspects, flow irregularities may be provided by thechannel geometry which yield aggregation or deposition of particulates,e.g., nanowires in desired regions. By way of example, and as shown inFIG. 5C, one may provide channels 520 that include coves 522 in thechannel geometry, or corners 524 at which will function as depositionzones 526 as a result of eddies or other recirculating flows thatcontain nanowires in these regions for extended times. Streamlines areindicated by the dashed arrows.

[0040] In a further aspect, one may employ other means forpreferentially depositing wires in certain locations. One such exampleinvolves producing standing wave patterns in the fluid containing thenanowires over the surface of the substrate. Such standing waves can beused to create regular periodic patterns of nanowires deposited on thesubstrate surface. FIG. 5D schematically illustrates a fluid channel 530and wave generator 532, as well as an exemplary deposition pattern forthe nanowires on the surface of the substrate. As shown, a series ofstanding rolls 534 is set up within a fluid containing channel thatyields periodic deposition of nanowires, e.g., in zones 536. Inaddition, interfering waves could be set up in other directions, e.g.,orthogonal to the first standing wave, to provide more preciselocalization of deposited wires. Wave generators that are particularlyuseful in accordance with this aspect of the invention includepiezoelectric elements that provide high frequency vibrations to thefluid within the channel.

[0041] Positioned and oriented nanowires are far more amenable tointegration with electrical elements in a controlled, high yieldfashion. In particular, by providing populations of positioned andoriented nanowires, e.g., populations 124-138, one can more preciselyselect locations for electrical contacts, in order to maximize thelikelihood of functional connection between nanowires and electricalcontacts or other elements. By way of example, if one has a populationof nanowires that are randomly dispersed within a relatively small area,but are oriented to be pointing substantially in one direction, one canprovide electrical contacts within that small area and spaced apart inthe direction of orientation by a distance that will likely be spannedby at least one nanowire. Such a distance can be selected to be lessthan the average size of the nanowires in the population of nanowires.To ensure greater likelihood of spanning the contacts, one could placethem at a distance that is less than 90% of the average length, lessthan 80% of the average length, less than 70% of the average length, andin some cases, less than 50% of the average length of the nanowires inthe population. Of course, the closer together the contacts, the morelikely it becomes that one or many nanowires will bridge the twoelectrical contacts. Although described in terms of two electricalcontacts, it will be appreciated that the nanowires may be integratedwith a wide variety of other elements, including multiple, e.g., morethan two electrical contacts, other circuit elements or nanoscalestructures or elements fabricated into or onto the substrate (see, e.g.,commonly owned Provisional U.S. patent application Ser. No. 60/392,205,filed Jun. 27, 2002 and incorporated herein by reference in its entiretyfor all purposes), structural elements, e.g., ridges, posts, walls,etc., optical elements, or virtually any other element that would beemployed in a device that comprises nanowires.

[0042] Integration of the populations of nanowires on the wafer withelectrical elements can either be concurrent with the immobilizationprocess or it can take place in a subsequent separate step. Inparticular, the wafer may be pre-patterned with electrical contacts,such that immobilization of nanowires in selected regions correspondingto the positions of the electrical contacts yields wires that bridge thecontacts. Alternatively, the electrical contacts may be patterned overthe nanowires (or at least portions of the nanowires) that areimmobilized on the wafer.

[0043] As shown in FIG. 2, a number of metallization patterns can beprovided on a wafer. As shown in FIG. 2, a wafer 100 that has nanowirepopulations 124-138 deposited thereon is subjected to further processingto deposit electrical elements onto it. As noted above, however,electrical elements may be prepatterned onto the substrate. Ametallization pattern is established on the substrate using conventionalphotolithographic processes, e.g., photolithographically defining anddeveloping a pattern in a resist coating over the substrate, followed bye.g., evaporative deposition or sputtering of metal electrodes in theopen regions. As shown, a photomask 202 that corresponds to the desiredelectrode pattern 204 is used in the photolithographic definition of theelectrode patterns. As can be seen, the wafer based process producesmultiple discrete devices (each corresponding to a square 206 in thephotomask). Once the electrodes are laid down on the substrate, the maskis removed to yield a wafer with multiple integrated devices 208, whereeach device includes a discrete pattern of electrodes 210 that areconnected by nanowires 212 within each population of nanowires. Asshown, the electrode patterns are targeted to be overlaid upon theregions where the different populations of nanowires are deposited, tomaximize the potential of accurate integration of the two elements.

[0044] As shown, the electrical contact patterns also employ elements ofefficiency. In particular, as shown, a common electrode 222 is providedfor all device elements in a discrete device. In particular, while anumber of nanowire based devices are provided, e.g., the wire connectionbetween electrode 224 and 222 and between electrode 220 and 222, bothelements share the common electrode 222. This permits the easierconnection of the electrical contacts for all of the elements within agiven device with other portions of an overall system. In the case ofoperable devices, it will be readily appreciated that each device mayinclude a single wire connection or may include multiple connections,e.g., as shown in FIG. 2. Further, these connections may be of the sametype, e.., wires of the same composition, or with surface treatmentsthat are the same, e.g., attached ligands, antibodies, nucleic acids,etc. (for sensor applications). Alternatively, each device may comprisemultiple different wire connections, e.g., wires that have a differentbasic composition or surface binding element. For a discussion of sensorbased applications of nanowire based devices, see, e.g., U.S.Provisional Patent Application No. 60/392,205, filed Jun. 27, 2002, andCui, et al., Science 293, 1289-1292 (2001), the full disclosures ofwhich are incorporated herein by reference in their entirety for allpurposes.

[0045] Again, as noted above, each metallization pattern 210 correspondsto an individual device. As shown in the expanded view, eachmetallization pattern 210 includes a series of patterned electricalcontacts/traces, e.g., contacts/traces 220, 222 etc. The pairs ofelectrical contacts, e.g., contact 220 and 222, are spaced apart fromeach other by a distance that has a desired likelihood of having adesired number of nanowires that span the two electrical contacts. Inparticular, if one has a population of nanowires where the averagelength of nanowires is approximately 10 μm, one can increase thelikelihood of one or more wires spanning two electrical contacts byplacing them less than 10 μm apart. The closer the electrical contactsare together, the more likely it will be that at least one nanowire sillspan the two contacts. Thus, in some cases, the electrical contacts willbe less than 5 μm apart, and in other cases, less than 1 μm apart.

[0046] As will be readily appreciated, the methods described herein arenot limited to single sets of nanowires oriented in a single direction,but can be used to provide substrates that include nanowires oriented inany desired direction. Such differently oriented nanowires can bepositioned at different locations on a substrate or substrate wafer, orthey can be provided in the same location, e.g., layered, so as toprovide arrays of crossed, but electrically or structurally couplednanowires. Alternatively, such layered structures may simply be used toprovide a three dimensional architecture for a device, e.g., where eachlayer of nanowires is separated by an intermediate layer.

[0047] For example, following immobilization and orientation ofnanowires in a first direction, the manifold element may be rotated andadditional nanowires immobilized and oriented on the surface of thesubstrate. The result is populations of nanowires positioned on asubstrate that are oriented in a first direction that overlap withpopulations of nanowires oriented in a different direction. Nanowiresthat are differently oriented may comprise the same composition or theymay comprise different compositions. For example, a first population ofsemiconductor nanowires that is p doped may be positioned and orientedin a first direction. A second population may be positioned and orientedorthogonally to the first set and may include n-doping. The resultingp-n junction could then be used for a variety of different applications,including, e.g., optoelectronic applications, memory and logicapplications, and the like, e.g., as discussed in Published PCTApplication Nos. WO 02/17362, WO 02/48701, and 01/03208, the fulldisclosures of which are hereby incorporated herein by reference intheir entirety for all purposes.

[0048] Bidirectional or multidirectional orientation of nanowires isschematically illustrated in FIG. 4. As shown in FIG. 4, fluidcontaining nanowires is flowed in one direction over the substratesurface region 400 where wire deposition is desired. This results in thedeposition and immobilization of wires 402 in this region where thewires are substantially longitudinally oriented in the direction offlow. Fluid containing wires are then flowed over the same substrateregion 400 in a different direction, e.g., orthogonal to the originaldirection of flow and orientation. This results in deposition andimmobilization of wires 404 on the same substrate region oriented in thedifferent direction. This will result in a certain number of cross wirejunctions 406 being formed on the substrate surface. By then addingelectrical contacts 408, 410, 412 and 414, either before or after theaddition of wires, one can establish integrated electrical crossjunctions, which may include wires of like or different composition,e.g., doping.

[0049] In an alternative arrangement, and as discussed above, integratednanowire junctions may be created from a first nanowire that isfabricated onto the surface of the substrate by more conventional means,e.g., e-beam lithography or the like (see, U.S. Provisional ApplicationNo. 60/392,205, previously incorporated herein by reference). Such“integrated circuits” may be readily combined with the free standingnanostructures in accordance with the present invention, as can otherintegrated circuit elements, e.g., elements that are fabricated into oronto the surface of the substrate prior to adding the nanostructureelement as described in the present invention. A second nanowire isinterfaced with the first using the flow based alignment methodsdescribed herein. By way of example, a thin nanowire element may befabricated from an SOI wafer where the relevant semiconductor isp-doped. An n-doped, free standing nanowire is then deposited across thefirst wire element to provide a p-n junction. A variety of differentjunction types may be created in this manner, including simple switches,etc. as described above.

[0050] As described above, the methods of the invention involve theimmobilization of nanowires onto a surface of a substrate. As usedherein, the term “immobilization” refers to the coupling of a nanowirewith the substrate surface, or chemical groups on that surface, suchthat the nanowire remains in position on the substrate surface despitebeing contacted by fluids, moving air or gas, etc. Immobilization may bepermanent or reversible. Typically, immobilization is the result ofchemical interaction between the surface or chemical groups on thesurface and the nanowires themselves, or chemical groups on thenanowires. Such interactions include, e.g., ionic interactions, covalentinteractions, hydrophobic or hydrophilic interactions, and electrostaticor magnetic interactions.

[0051] In the case of certain substrates and nanowires, the existingsurfaces of the substrate and the nanowire may provide sufficientattraction between the substrate and the nanowire to provideimmobilization. For example, where the nanowires and substrate surfaceare generally hydrophilic, one could dispose the nanowires in ahydrophobic solvent to contact them with the surface. As a result, thefavored reaction would be for the nanowires to associate with thesubstrate surface, resulting in immobilization. Alternatively, and inparticularly preferred aspects, one may provide surfacefunctionalization on one or both of the substrate and/or the nanowirethat facilitates coupling between the two.

[0052] In functionalizing the substrate surface, where suchfunctionalization is necessary or desired, one may provide the abilityto couple the nanowires to an entire substrate surface and rely upon themasking step to selectively position nanowires, or one may also provideonly selected regions of functionalized surfaces to further selectivelyposition nanowires on the surface. In particular, one may functionalizeonly first selected regions on the substrate or wafer. Then, by maskingoff other selected regions that include portions of the functionalizedregions, one can further control how nanowires are coupled to thesurface of the substrate. FIG. 3 illustrates such a process using thesame manifold 102 for surface functionalization followed by nanowiredeposition. As shown, manifold 102 is placed over wafer 100, andappropriate functionalization chemistry is directed through the channelsof the manifold. This results in derivatized surface regions thatcorrespond to channels 104-118. The manifold is then rotated, e.g., 90°,and nanowire containing fluid is directed through the manifold. Becauseonly a portion of the surface which the nanowires contact isfunctionalized, the nanowires will be positioned and orientedsubstantially only in those regions. When the manifold is removed, ityields a substrate in which nanowires are only immobilized in selectedsmall regions, e.g., regions 324, 326, etc., that were bothfunctionalized, and exposed to nanowires. This provides for even moreprecise control over positioning of the wires. For example, one cantarget the functionalization to provide more precise localization ofnanowires in the desired regions, such as functionalizing the surface ofthe electrical contacts, but no other portions of the substrate surface,in order to assure that the wires immobilize only to the electricalcontacts, or in the regions where electrical contacts are to beprovided.

[0053] Functionalization of the surface may be carried out by a varietyof means. For example, as discussed above, functionalization may bedirected at an entire substrate surface, or it may be patterned orchemically templated onto the substrate surface. As used herein, theterm “chemical template” generally refers to the deposition and/orreaction upon a substrate surface of a template that is defined bychemical modification of that substrate surface. In particular, chemicalmodification of the surface in selected regions will make it more likelythat a nanostructure will localize to a particular region, e.g., adesired region, than in another region, e.g., an undesired region.Chemical modification can be positive modification, e.g., the region ofmodification provides enhanced affinity of the nanostructure to thesubstrate, or it can be negative, e.g., it provides a repulsing effectsuch that nanostructures are unlikely to localize in the particularregion. Chemical modifications include any of a variety of differentsurface treatments that are well known in the art of surface chemistry,including coupling of active groups that are capable of bonding to orotherwise associating with the nanostructures or with chemical groupsdisposed upon those nanostructures. The functional chemical groupspresented may interact with the nanostructures via affinityinteractions, ionic interactions, hydrophobic and/or hydrophilicinteractions.

[0054] As noted above, the substrate may comprise a bare substrate ormay include other elements, including other device elements and/or othernanostructures, e.g., electrodes, nanowires, circuit elements, etc. Thechemical moieties may be an element of the substrate, or they may becoupled, either directly or through a linker molecule, or otherwiseprovided upon the surface of the substrate in the desired pattern or atthe desired locations or regions of the substrate's surface.

[0055] One arrangement for capturing nanostructures involves formingsurfaces that comprise regions that selectively attract nanostructures.For example, —NH₂ moieties can be presented in a particular pattern at asurface, and that pattern will attract nanowires or nanotubes havingsurface functionality attractive to amines. This same surfacefunctionality is also optionally used to generate an ionic attractionwhereby surface amines are exposed to an acidic environment resulting ina predominantly positively charged surface, e.g., populated with NH₃ ⁺groups that can attract negatively charged nanostructure surfaces orrepel like charged materials. Surfaces can be patterned using knowntechniques such as electron-beam patterning, soft-lithography, or thelike. See also, International Patent Publication No. WO 96/29629,published Jul. 26, 1996, and U.S. Pat. No. 5,512,131, issued Apr. 30,1996.

[0056] Templates may have inherent affinity toward nanostructures, ormay be provided such that the affinity can be accentuated. For example,in preferred aspects, chemical templates are generated by providingprotected functional groups over the surface of the substrate upon whichthe nanostructures are going to be provided. Desired portions or regionsof the substrate surface are then deprotected, e.g., the protectinggroups are removed or transformed, to yield an active site to whichnanostructures will bind or otherwise be localized. As alluded to above,the regions of the substrate that are deprotected may comprise a basicsubstrate surface, e.g., a SiO₂ substrate, or they may include otherelements, including functional elements, on the surface of a basicsubstrate. For example, a chemical template may define regions only onthe surfaces of electrical contacts that are present on a basicsubstrate, and not elsewhere on the substrate surface, so as to increasethe likelihood that nanostructures, e.g., nanowire(s), will be coupledto those electrodes, and nowhere else.

[0057] In a first aspect, photodeprotection is used to provide achemical template for directed positioning of nanostructures. Inparticular, a substrate to which nanostructures are to be coupled, boundor otherwise associated, is treated to provide a layer of chemicalmoieties that include active functional chemical groups that wouldinteract, e.g., bind, to a nanostructure, but for the presence of aprotecting group coupled to that active group. In accordance with thisaspect of the invention, the protecting group provided on the activegroup is a photolabile protecting group. Specifically, in order toactivate the molecules on the surface of the substrate, one must exposethe photolabile protecting group to light of a desired wavelength, toremove the protecting group and yield the active chemical moiety withwhich a nanostructure may interact/bind. By selectively exposing desiredregions of the substrate, one can selectively activate a pattern ofregions on that surface and drive the selective positioning ofnanostructures accordingly. Such selective exposure can be carried outusing standard photolithographic techniques, e.g., mask-based exposure,laser writing, e-beam lithography, etc. that are very well known in theart.

[0058] A wide variety of photolabile protecting groups and theirassociated linkage chemistries, e.g., that couple other elements tosurfaces, once activated, are well known in the art, and have been usedextensively in the directed positioning of chemical elements onsubstrate surfaces. By way of example, in at least one aspect of theinvention, amino or hydroxyl terminated organosilane linker moleculesare provided coupled to the substrate surface. The linker group iscapped by a protecting group that is cleaved or rendered cleavable uponexposure to light of a desired wavelength. Examples of known photolabileprotecting groups include nitroveratryloxycarbonyl protecting groups,such as NVOC and MeNVOC, as well as nitropiperonyloxycarbonyl protectinggroups, such as NPOC and MeNPOC, and others, e.g., PyMOC. The use ofthese protecting groups and others in photolithographic activation ofsurfaces is described in, e.g., U.S. Pat. Nos. 5,489,678 and 6,147,205,the complete disclosures of which are hereby incorporated herein byreference in their entirety for all purposes.

[0059] In alternative arrangements, functional groups may be in anionizable form, such that under certain conditions, e.g., low or highpH, the functional group has substantial affinity for the nanostructure,e.g., a strong positive or negative charge, while under differentenvironmental conditions, the affinity is substantially lower, or iseven negative.

[0060] In certain aspects, the organosilane polymer is terminated with ahydrophilic moiety. In such cases, the natural affinity of the nanowirecomponents, e.g., for semiconductor nanowires, to the hydrophilic moietyprovides the selectivity of binding in the overall positioning process.Examples of linkers including such hydrophilic terminators include,e.g., (hydroxy/amino) propyltriethoxy silane derivatives andpoly(hydroxy/amino)propyltriethoxysilane derivatives. To take advantageof an increase in hydrophilicity, protecting groups for this particularembodiment would be generally hydrophobic in nature. Cleavage would thenyield an increase in hydrophilicity at the desired location. Use ofrelatively hydrophilic and hydrophobic surface templates has beendescribed for use in, e.g., in situ chemical synthesis for biochemicalmicroarrays (see, e.g., U.S. Pat. No. 5,985,551, to Brennan et al.).

[0061] In this case, following exposure of the desired regions to exposethe hydrophilic moiety in the desired regions for couplingnanostructures, a fluidic suspension of nanostructures is flowed over orotherwise contacted with the entire substrate. The affinity of thenanostructures, e.g., semiconductor nanostructures, for the hydrophilicregions provides for the preferential localization of the nanowires inthe deprotected regions. Such regions may include regions between andincluding electrical contacts, or between electrical contact(s) andother nanostructures, or regions on substrates where subsequentadditional elements will be patterned to contact the nanostructures sodeposited.

[0062] While described in terms of hydrophilic affinity, it will beappreciated that a variety of different interactions may be exploited inthe attraction and/or repulsion of nanostructures within the selectedpattern, including hydrophobic interactions, e.g., in regions where itis not desired to have binding, combined hydrophobic/hydrophilicinteractions, specific molecular affinity interactions, e.g.,antibody:antigen, aviden:biotin, nucleic acid hybridization,, or ionicinteractions.

[0063] In the cases of affinity interactions (and even in othernon-affinity cases, where higher efficiency coupling is desired), it maybe necessary or desirable to provide a functional group on the nanowireto permit the desired interaction between the nanowire and thesubstrate, e.g., a complementary molecule to that disposed on thesubstrate surface. In such cases, and particularly with reference tosemiconductor nanowires, e.g., silicon nanowires, derivatization of thenanowire may be carried out according to methods used to derivatize thesubstrate surface of like make-up. For example, nanowires may besilanized for attachment to the substrate surface, either directly orthrough an intermediate group. In particular, in the same fashiondescribed for derivatizing the surface of the substrate, supra, onecould derivatize the nanowire itself. Such derivatization could includeaddition of affinity molecules, hydrophilic or hydrophobic groups, ionicgroups, etc. as desired to improve efficiency of the positioningprocess. In addition, functionalization of the nanowire providesfacility in adding additional components to the nanowire element, i.e.,for attachment of biomolecules for biosensor applications (see, e.g.,U.S. patent application Ser. No. 60/392,205, previously incorporatedherein). Thus, in certain cases both the nanowire and substrate may bederivatized to facilitate binding and improve efficiency of thepositioning process.

[0064] Additionally, or alternatively to the photo-deprotection processdescribed above, patterning of a surface for nanostructure attachmentmay utilize chemical deprotection methods, e.g., acid deprotection. Aciddeprotection generally utilizes acid labile protecting groups in placeof the photolabile protecting groups described above. Directed exposureof regions to acid may be accomplished through mechanically directedmeans, e.g., channeling acid to the desired regions while preventing theacid from reaching other regions. Such mechanical means can include theuse of channel blocks mated with the substrate, template masks. However,such methods often yield low resolution due to the difficulty in sealingthe channel block to the substrate surface. Other mechanical methodsinclude ink jet printing methods, microcontact printing methods, etc.

[0065] For modification of electrodes to increase affinity, one couldprovide the patterned electrodes with a thin gold layer as the chemicalmoiety to increase affinity, and treat the nanostructures with thiolterminated organosilanes. The thiolated nanostructure would then bindpreferentially to the gold electrode.

[0066] Alternatively, photoresist layers are used to generate amechanical stencil or mask for subsequent acid exposure. In particular,a resist is coated on a substrate that includes acid labile protectinggroup capping the functional groups. The resist is exposed anddeveloped, e.g., removed, in the desired regions and the exposedportions of the substrate are subjected to acid deprotection while theunexposed regions are not. In still more preferred aspects, an acidgenerating resist is used, where exposure of the resist in the desiredlocations results in generation of an acid which in turn deprotects thefunctional groups in those desired locations. This latter method has anadded advantage of reducing the number of required process steps, inthat the exposure and acid deprotection steps are concurrent.

[0067] Examples of both acid labile protecting groups and acidgenerating photoresists are well known in the art and include, e.g., DMT(dimethoxytrityl) and its derivatives, as well as acid generating resistlayers that are generally commercially available.

[0068] Although described primarily in terms of photolithographicpatterning techniques, it will be appreciated that other patterningtechniques, such as microcontact printing techniques, laser ablativetechniques (either direct or in conjunction with a resist layer, i.e.,PMMA), and the like may be employed in the patterning steps. Suchmethods are generally well known in the art and are described in, e.g.,U.S. Pat. No. 6,180,239 to Whitesides et al, and U.S. Pat. No. 5,500,071to Swedberg et al.

[0069] For other applications, different protecting group types may beemployed, e.g., allyloxycarbonyl (ALLOC), fluorenylmethoxycarbonyl(FMOC), —NH-FMOC groups, t-butyl esters, t-butyl ethers, and the like.Various exemplary protecting groups are described in, for example,Atherton et al., (1989) Solid Phase Peptide Synthesis, IRL Press, andGreene, et al. (1991) Protective Groups In Organic Chemistry, 2nd Ed.,John Wiley & Sons, New York, N.Y.

[0070] For the steps of selectively patterning nanowires onto thesurface, through the use of a masking element, while in preferredaspects, the masking element or manifold is provided as a separateelement or layer that is removably placed against the substrate surface,it will be appreciated that this element may be fabricated onto thesurface of the substrate, e.g., in the same fashion as described withreference to chemical templating of the substrate surface. Further, thismasking element may remain permanently on the surface of the substrate,or it may be removed through subsequent processing of the substrate. Inparticular, a manifold element may be fabricated onto a substrate orsubstrate wafer surface by coating a layer of material, e.g., apolymeric resist layer on the substrate. In preferred aspects, polymericresists, and preferably photoresists are spin coated onto wafersurfaces. As described above, the substrate may include electricalcontacts pre-patterned onto the surface of the wafer. Similarly, thesurface may be pre-functionalized in first selected regions for couplingto nanowires, as described above.

[0071] Following coating of the layer that is to form the manifold ormasking layer onto the substrate surface, passages are defined throughthat layer, typically as troughs, trenches or fluid channels in thelayer to provide fluid access to the surface of the substrate. By usinga photoresist as the masking layer, one can simply use the recommendedexposure and development processes for the resist used to define thepassages. Once the masking layer is defined on the surface of thesubstrate, fluid containing the nanowires is flowed over the substrateand/or specifically through the defined channels or troughs in thedesired direction in order to allow the nanowires to be immobilized onthe substrate surface in the desired orientation. As will beappreciated, enclosed or sealed channels are generally preferred forflowing nanowires in a desired direction. As such, in preferred aspects,an additional cover layer is optionally and preferably disposed over themasking layer to provide flow channels, like with a manifold. Whileeither positive or negative resists may generally be employed inaccordance with the invention, for use in templating, it will begenerally desirable to use positive photoresists, as they are less proneto swelling in aqueous solutions or in ethanol, which is often employedas the fluid carrier for nanostructures, e.g., nanowires. Positiveresists additionally provide better adhesion to many substrate layertypes, e.g., silicon, and provide greater mechanical strength. Thisallows for more precise templating or masking in the positioning ofnanowires. As noted, this layer may be removed in subsequent steps or itmay be allowed to remain on the overall device to provide additionalstructural features, e.g., insulation, moisture barriers, fluidicconduits, etc. A wide variety of different positive and negative resistsare generally commercially available, e.g., from DuPont, i.e., DuPont8000 series resists.

[0072] Once the nanowires are deposited, the masking layer may beremoved from the substrate to allow for additional processing.Alternatively, where various integration elements are prepositioned onthe wafer or substrate, it may not be necessary to remove the maskinglayer. In fact, in some cases, the masking layer may provide a barrieror insulation between electrical or fluidic elements of a device. In thecase of such masking layers, it will be appreciated that any of avariety of resist layers are readily commercially available for thisprocess, including, e.g., polyimide or PMMA based resists, or any of avariety of resists that are generally commercially available.

[0073] One of the advantages of the invention is its applicability tomanufacturing on a commercial scale. In accordance with this advantage,FIG. 6 schematically illustrates an overall system that may be used incommercial scale alignment and deposition of nanowires onto substratesand subsequent device integration. As shown, the system 600 includes asource of nanowire containing fluid 602. A pump 604 delivers the fluidto the inlet port 606 of a deposition module 608 which would typicallyinclude a base substrate 610 onto which nanowires are to be deposited,and a manifold element 612 which directs the flowing nanowires toselected regions on the surface of the substrate to which the manifoldis mated. Following contact with the substrate, the fluid exits themanifold 612 through outlet port 614, where the fluid and the nanowiresstill contained therein are reclaimed, e.g., in a reclamation vessel(not shown) or recycled back into source 602 (as shown). Typically, themodule 606 may be multiplexed either in parallel, e.g., as shown bymodule 616, or in series, as shown by module 618, provided there is asufficient concentration of nanowires in the fluid, in order to increasethe throughput of the deposition process.

EXAMPLES Example 1

[0074] Controlled Positioning and Flow Alignment of Nanowires on a WaferScale

[0075] Nanowires were positioned and oriented on a substrate andsubsequently integrated with electrical connections in accordance withthe invention.

[0076] Silicon nanowires used for flow alignment were synthesized bygold cluster mediated chemical vapor deposition methods, and theresulting nanowires were suspended in ethanol solution viaultrasonication.

[0077] A poly(dimethylsiloxane) (PDMS) stamp, e.g., as shown in FIG. 1,was fabricated by photolithography. The PDMS stamp had a three-inchdiameter, with eight parallel channels spaced 7 mm apart with eachchannel having a width of 500 μm, and a depth of ˜200 μm.

[0078] A silicon substrate wafer (surface oxidized, 600 nm oxide) to beused in flow assembly was functionalized with an NH₂-terminatedself-assembled monolayer (SAM) by immersion in a 1 mM chloroformsolution of 3-aminopropyltriethoxysilane (APTES) for 30 min, followed byheating at 110° C. for 10 min.

[0079] Alignment of nanowires was performed by conforming the PDMS stampto the functionalized surface of the silicon substrate. The ethanolsolution of nanowires was flowed into the parallel channels of the stampthrough one port (inlet) and out through the other port. Flow was eitherinduced by gravity, e.g., tilting the substrate to ˜40°, or throughapplication of a positive pressure to the inlet port.

[0080] Once the nanowire solution was delivered through the stamp, thePDMS stamp was removed, and the surface of the substrate wafer wascoated with a photoresist. FIG. 7 shows an SEM image of flow alignednanowires immobilized on a substrate. As can be seen, a substantialmajority of the nanowires are substantially longitudinally oriented in asingle direction in the direction of flow during the deposition process.

[0081] By superimposing virtual or postulated pairs of electricalcontacts over the oriented nanowires, one can estimate the efficacy ofthe fabrication process in producing functioning nanowire containingdevices, e.g., devices in which one or more nanowires connects a pair ofelectrical contacts. A virtual electrical contact pattern overlaid onthe oriented nanowires is shown in FIG. 8A. Examination of overlay inFIG. 8A allows for an estimate of 0, 1, 2 and 3 wire connections betweenelectrode pairs. FIG. 8B provides a plot of the distribution ofconnections in the estimated devices of FIG. 8A. As can be seen,functional device yield, e.g., percentage of devices showing one or moreconnection between a pair of electrical contacts, is approximately 75%.

[0082] Photolithography was used to selectively remove portions of thephotoresist, and electron-beam evaporation was performed to define themetal contacts on to the nanowires in selected locations on thesubstrate surface. The pattern of electrodes was as shown in FIG. 1.FIG. 9 illustrates the overall device, as well as expanded views of theelectrodes and nanowire connections between electrode pairs. Electrodepairs are made up between the common central electrode and each of theseparate orthogonally oriented electrodes. Each connected electrodepair, e.g., connection between the central electrode and an orthogonalelectrode, represents an operation element of a nanowire based device.

[0083] Although described in considerable detail above, it will beappreciated that various modifications may be made to theabove-described invention, while still practicing the invention as it isdelineated in the appended claims. All publications and patent documentscited herein are hereby incorporated herein by reference in theirentirety for all purposes to the same extent as if each such documentwas individually incorporated herein.

What is claimed is:
 1. A method of depositing a population of nanowireson a surface substantially in a desired orientation, comprising: flowinga first fluid containing nanowires over the surface in a firstdirection, the first direction being parallel to a desired longitudinalorientation of the nanowires; and permitting a population of nanowiresin the first fluid to become immobilized onto the surface, alongitudinal dimension of the nanowires from the first fluid beingsubstantially oriented in the first direction.
 2. The method of claim 1,wherein the flowing step comprises providing a microscale fluid channelon the surface of the substrate and flowing the first fluid through thefluid conduit in the first direction.
 3. The method of claim 1,comprising providing a plurality of fluid channels over differentregions of the surface of the substrate, and flowing the first fluidthrough each of the fluid conduits in the first direction.
 4. The methodof claim 2, wherein the microscale fluid channel comprises one or moreof a widened region, a shallow region, and a cove, the nanowirespreferentially immobilizing in the one or more widened region, shallowregion and cove.
 5. The method of claim 2, wherein the step of providinga microscale fluid channel on the substrate surface comprises providinga manifold having a first groove disposed in its first surface, andmating the first surface of the manifold with the surface of thesubstrate to define a first enclosed channel on the first surface of thesubstrate.
 6. The method of claim 2, wherein the step of providing amicroscale fluid channel on the substrate surface comprises providing alayer of polymeric material on the substrate surface and defining themicroscale fluid channel in the layer of polymeric material to providefluidic communication to at least a portion of the substrate surface. 7.The method of claim 6, wherein the layer of polymeric material comprisesa photoresist, and the step of defining the microscale fluidic channelcomprises exposing a portion of the layer of photoresist and developingthe layer of photoresist to define the microscale fluidic channel. 8.The method of claim 6, further comprising providing a cover layer overthe layer of polymeric material to seal and enclose the microscalefluidic channel, the cover layer comprising at least a first portdisposed therethrough and positioned to provide fluid access to themicroscale fluidic channel.
 9. The method of claim 8, wherein thephotoresist comprises a positive photoresist.
 10. The method of claim 1,wherein the permitting step comprises providing the first surface of thesubstrate as a functionalized first surface that is capable of bindingthe nanowires from the first fluid.
 11. The method of claim 10, whereinthe step of providing the first surface of the substrate as afunctionalized first surface comprises providing functional groups ononly a portion of the first surface.
 12. The method of claim 11, whereinthe portion of the first surface comprises one or more electricalcontacts.
 13. The method of claim 11, wherein the functional groupscomprise protectable or deprotectable functional groups.
 14. The methodof claim 13, wherein the functional groups comprise photodeprotectablefunctional groups.
 15. The method of claim 1, further comprising thestep of providing at least first and second electrical contacts on thesurface of the substrate, the first and second electrical contacts beingseparated by a first distance in the first direction on the firstsubstrate, the first distance being less than an average length of thenanowires in the first fluid containing nanowires.
 16. The method ofclaim 15, wherein the first and second electrical contacts are providedon the surface of the substrate before flowing the fluid containing thenanowires over the first surface.
 17. The method of claim 15, whereinthe first and second electrical contacts are provided on the surface ofthe substrate after the nanowires have been permitted to be immobilizedon the first surface of the substrate.
 18. The method of claim 1,further comprising flowing a second fluid containing nanowires over thesurface in a second direction different from the first direction, andpermitting the nanowires in the second fluid to become immobilized ontothe surface whereby a longitudinal dimension of the nanowires from thesecond fluid being substantially oriented in the second direction. 19.The method of claim 18, wherein the nanowires from the first fluidoriented substantially longitudinally in the first direction areimmobilized to at least a portion of a same region of the surface of thesubstrate as nanowires from the second fluid oriented substantiallylongitudinally in the second direction.
 20. The method of claim 18,wherein the nanowires from the first fluid oriented substantiallylongitudinally in the first direction are immobilized to a differentregion of the surface of the substrate as nanowires from the secondfluid oriented substantially longitudinally in the second direction. 21.The method of claim 1, further comprising: functionalizing at least afirst portion of the surface of the substrate, prior to flowing thefirst fluid containing the nanowires over the first surface in the firstdirection, whereby the nanowires immobilize to the first portion of thesurface of the substrate that has been functionalized.
 22. The method ofclaim 21, wherein the first fluid is flowed over a second portion of thesurface of the substrate, the first portion of the surface and thesecond portion of the surface at least partially overlapping.
 23. Themethod of claim 21, wherein the first surface of the substrate comprisesat least one other circuit element to which nanowires are to be coupled.24. The method of claim 23, wherein the at least one other circuitelement comprises at least a first pair of electrical contacts.
 25. Themethod of claim 24, wherein the first pair of electrical contactscomprises first and second metal contact regions on the surface of thesubstrate.
 26. The method of claim 23, wherein the at least one othercircuit element comprises a nanowire circuit element.
 27. The method ofclaim 26, wherein the nanowire circuit element comprises dopingdifferent from doping in the nanowires in the first fluid.
 28. Themethod of claim 23, wherein the circuit element comprises an integratedcircuit element disposed on the substrate.
 29. The method of claim 18,wherein the integrated circuit element comprises a nanoscale circuitelement.
 30. A method of positioning nanowires in one or morepredetermined regions on a substrate, comprising: providing a substratehaving a first surface; overlaying the first surface with a mask, themask providing fluid access to one or more first predetermined regionson the first surface, but not to one or more second predeterminedregions on the surface of the substrate; flowing fluid containingnanowires through the mask and into contact with the first predeterminedregions of the substrate surface; and permitting the nanowires containedin the nanowire containing fluid to immobilize in the firstpredetermined regions of the surface of the substrate.
 31. The method ofclaim 30, wherein the step of providing the substrate comprising thefirst surface comprises providing the first surface as a functionalizedsurface capable of binding to the nanowires.
 32. The method of claim 30,wherein the flowing step comprises flowing the fluid containing thenanowires over the first predetermined regions in a first direction tocause the nanowires to immobilize in the first predetermined regions onthe surface of the substrate longitudinally oriented substantially inthe first direction.
 33. The method of claim 30, further comprisingproviding at least first and second electrical contacts in the one ormore first predetermined regions, whereby one or more nanowiresimmobilize in contact with both the first and second electrical contactsin the first predetermined region.
 34. The method of claim 33, whereinthe first and second electrical contacts are provided in the firstpredetermined regions of the substrate before the nanowires areimmobilized in the first predetermined regions.
 35. The method of claim33, wherein the first and second electrical contacts are provided in thefirst predetermined regions of the substrate after the nanowires areimmobilized in the first predetermined regions.
 36. A population ofnanowires immobilized on a planar surface of a substrate, the populationof nanowires being substantially longitudinally oriented in a firstdirection parallel to the planar surface.
 37. The population ofnanowires of claim 36, comprising a plurality of discrete sets ofnanowires immobilized on separate regions of the surface of the firstsubstrate, the nanowires in each separate region being substantiallylongitudinally oriented in a selected direction.
 38. The population ofnanowires of claim 36, wherein the first substrate at least first andsecond electrical contacts disposed thereon, the first and secondcontacts being positioned sufficiently proximal to each other in thefirst direction, such that at least one nanowire in the population ofnanowires is simultaneously contacting both of the first and secondelectrical contacts.
 39. The population of nanowires of claim 38,wherein the first and second electrical contacts are deposited over atleast a portion of the at least one nanowire.
 40. A population ofnanowires immobilized on a surface of a substrate, comprising: a firstset of nanowires immobilized in a first selected region of the surfaceof the substrate; and a second set of nanowires immobilized in a secondselected region of the surface of the substrate, the second selectedregion being separate from the first selected region.
 41. A nanowirebased device, comprising: at least a first population of nanowiresimmobilized in at least a first region of a surface of a substrate, thefirst population of nanowires being substantially longitudinallyoriented in a first direction; at least first and second electricalcontacts disposed on the first region of the surface of the substrate;and wherein the first and second electrical contacts are separated fromeach other on the first surface of the substrate in the first directionby a less than an average length of the nanowires in the firstpopulation of nanowires.
 42. The nanowire based device of claim 41,wherein at least one nanowire in the population of nanowires ispositioned in contact with both the first and second electricalcontacts.
 43. The nanowire based device of claim 41, wherein the firstand second electrical contacts are separated by a distance that is lessthan 90% of the average length of nanowires in the first population ofnanowires.
 44. The nanowire based device of claim 41, wherein the firstand second electrical contacts are separated by a distance that is lessthan 80% of the average length of nanowires in the first population ofnanowires.
 45. The nanowire based device of claim 41, wherein the firstand second electrical contacts are separated by a distance that is lessthan 50% of the average length of nanowires in the first population ofnanowires.
 46. The nanowire based device of claim 45, wherein the firstand second electrical contacts are separated by a distance that is lessthan 10 μm.
 47. The nanowire based device of claim 46, wherein the firstand second electrical contacts are separated by a distance that is lessthan 1 μm.
 48. The nanowire based device of claim 46, further comprisingat least third and fourth electrical contacts separate from the firstand second electrical contacts, and disposed on the first region of thefirst surface wherein the third and fourth electrical contacts areseparated from each other on the first surface of the substrate in thefirst direction by less than an average length of the nanowires in thepopulation of nanowires.
 49. The nanowire based device of claim 46,further comprising: at least a second population of nanowiresimmobilized in at least a second region of the surface of the substrate,the second population of nanowires being substantially longitudinallyoriented in a second direction; at least third and fourth electricalcontacts disposed on the second region of the surface of the substrate;and wherein the third and fourth electrical contacts are separated fromeach other on the first surface of the substrate in the second directionby a less than (1 μm/distance that is less than an average length of thenanowires in the population of nanowires).
 50. The nanowire based deviceof claim 46, further comprising: at least a second population ofnanowires immobilized in at least a second region of the surface of thesubstrate, the second population of nanowires being substantiallylongitudinally oriented in a second direction; at least third and fourthelectrical contacts separate from the first and second electricalcontacts, and disposed on he first region of the first surface whereinthe third and fourth electrical contacts are separated from each otheron the first surface of the substrate in the first direction by lessthan (1 μm/a distance that is less than an average length of thenanowires in the first population of nanowires); and at least fifth andsixth electrical contacts disposed on the second region of the surfaceof the substrate, wherein the fifth and sixth electrical contacts areseparated from each other on the first surface of the substrate in thesecond direction by a less than (1 μm/distance that is less than anaverage length of the nanowires in the second population of nanowires).51. A substrate comprising a plurality of populations of nanowiresdeposited upon a first surface of said substrate; and wherein each ofthe populations of nanowires is deposited and immobilized in a separatediscrete region of the surface of the substrate.
 52. The substrate ofclaim 51, further comprising at least a first pair of electricalcontacts deposited on the surface of the substrate, the first pair ofelectrical contacts being positioned to be in electrical contact withwires in at least a first of the plurality of nanowire populations. 53.A system for orienting nanowires on a surface of a substrate,comprising: a substrate having a first surface; a fluid channel disposedon the first surface; and a fluid direction system coupled to the firstchannel and coupled to a source of fluid containing nanowires, forflowing the fluid containing nanowires in a first direction through thefirst fluid channel.
 54. The system of claim 53, wherein the fluidchannel is defined in a first surface of a manifold, the first surfaceof the manifold being mated to the first surface of the substrate todispose the fluid channel on the first surface of the substrate.
 55. Thesystem of claim 54, further comprising at least a second fluid channeldisposed on the first surface of the substrate.
 56. The system of claim55, wherein the first and second fluid channels are fluidly coupled to afirst fluid inlet port, the first fluid inlet port being fluidly coupledto the source of fluid containing nanowires.
 57. A system forpositioning nanowires on a substrate, comprising: a substrate having afirst surface; a masking element disposed over the first surface andproviding one or more fluid passages to one or more discrete regions ofthe first surface; a source of fluid containing nanowires fluidlycoupled to the one or more fluid passages on the masking element; and afluid direction system for delivering fluid from the fluid source to theone or more fluid passages.
 58. The system of claim 57, wherein themasking element comprises a manifold having a plurality of fluidchannels disposed therein, the plurality of fluid channels having as atleast one wall of the one or more fluid channels the one or more regionsof the surface of the substrate, the fluid channels providing the one ormore fluid passages to the first surface of the substrate.
 59. Thesystem of claim 58, wherein one or more of the fluid channels comprisesa widened region corresponding to a position on the surface of thesubstrate where it is desired to position nanowires, the widened regionproviding longer residence time within the wider region for a fluidflowing through the one or more fluid channels.
 60. The system of claim58, wherein the one or more fluid channels comprises a thinned regionthat provides a shorter average diffusion distance between a fluidflowing through the thinned region of the one or more channels and thesurface of the substrate at the thinned region of the one or more fluidchannels.
 61. The system of claim 58, wherein the manifold comprises aflexible material.
 62. The system of claim 61, wherein the flexiblematerial comprises a polymeric material.
 63. The system of claim 61,wherein the flexible material comprises PDMS.
 64. A method ofpositioning nanostructures on a surface of a substrate, comprising:contacting the surface of the substrate with a fluid containing thenanostructures; establishing a standing wave through the fluid, thestanding wave localizing nanostructures preferentially in first selectedregions of the surface of the substrate and not in second selectedregions of the substrate.